Process for making permanently backed shell molds



United States Patent 3,446,265 PROCESS FOR MAKING PERMANENTLY BACKEDSHELL MOLDS Ronald H. Buck, Jr., Farmington, Mich., assignor to EatonYale & Towne Inc., Cleveland, Ohio, a corporation of Ohio N0 Drawing.Filed May 17, 1966, Ser. No. 550,632 Int. Cl. B22c 13/08 U.S. Cl. 164218 Claims ABSTRACT OF THE DISCLOSURE A process for making permanentlybacked shell molds by which a particulated refractory materialcontaining a resin binding agent is introduced under controlled pressureinto a cavity defined by a heated pattern and metal backing member whichare vibrated at high frequency during the injection of the refractorymaterial and continuously vibrated thereafter until a partial curing ofthe binding agent occurs.

This invention relates to the art of shell-lined, permanently backedmolds and to a method of casting articles with such molds.

In recent years, shell molding techniques have substantially advancedthe art of metal founding. In this process a sand and thermosettingresin mixture is placed on a pattern which is heated to cure and hardenthe resin. Thus, a shell mold of resin-bonded sand is produced over thepattern. Final baking produces the finished shell mold which is suitablybacked up by shot or by solid members prior to the castingoperation.

More recently, shell molds have been formed by injecting a suitablemixture of sand and resin into a cavity defined by a pattern positionedin closely spaced relation to a backing member. After final baking andremoval of the pattern, the shell mold with the backing member is usedfor casting.

The advantages of the shell-molding process are that the shells producevery little gas during the casting operation and, in addition, molddraft angles can be reduced so that articles can be cast in such moldswith greater precision and very good surface smoothness as compared withearlier methods.

A problem encountered with shell molding processes heretofore used isthat the quality of the casting surface of the shell has not beencontrollable with the highest degree of accuracy so that casting finishhas not been consistently uniform, especially for articles of thincrosssection and intricate configuration. Furthermore, when usingpermanent backing members in such processes, difiiculties areencountered in properly effecting uniformity of density in the shellcasting surface and in effecting proper flow during injection of thesand-resin mixture into intricate shaped cavities.

The present invention provides a method of making permanently backedcasting shells which provide a superior casting surface finish byaccurately controlling the uniformity of density in the casting surfaceof the shell mold.

The present invention provides a method for making shell molds by usingan effective combination of sand blowing and vibration for tamping thesand-resin mixture within the shell-forming cavity so that the densityof 3,446,265 Patented May 27, 1969 the shell is uniform throughout theshell-casting surface while using a minimum amount of resin.

The present invention further provides for the making of thin shellmolds having intricate and complex shellcasting surfaces of uniformdensity.

In a further aspect of the invention, permanently backed shell moldsproduced by this invention may be filled with molten metal having ahigher effective metal head producing castings of thin cross-section andintricate configurations. Also, increasing the effective metal head,either by vacuum, positive gas pressure, or by a mechanical means,provides for better mold fill without increasing the static metal headso that the length of sprue necessary to fill a given mold cavity isreduced. Also, a higher metl velocity is obtained, thereby makingabetter utilization of fluid life without resorting to excessivesuperh'eat.

In accordance with this invention, permanently backed, thin shell-linedmolds are formed by preparing a charge of dry, free-flowing sand whichis coated'with a thermosetting resin and injecting such material underpressure into and while vibrating a shell-forming cavity so that uponsubsequent curing of the thermosetting resin, the shell liner density isuniform throughout the casting surface thereof and such surface isexceptionally smooth and has high porosity.

In the present invention the shell-forming material consists of sand ofa uniform fineness which is dry, freeflowing and the particles of whichare coated with a thermosetting resin. The grain size usable in thisinvention may be in the range of -180 AFS but preferably within therange of l20150 AFS grain fineness. The sand particles are uniformlycoated with a thermosetting resin such as the phenolics or theurea-formaldehydes, although other such resins may be used.

In the present invention, the amount of resin is carefully limited to0.73.0% by weight of the sand. When utilized with a specific sand grainsize described above, improved shells are provided giving superiorcasting surface finish.

In general, as the grain fineness number increases, resin content isalso increased because of the greater surface area of the finer grains.Total surface area is therefore correlated to grain fineness number.Resin is primarily necessary for mold liner strength. Therefore, a 150AFS sand would require approximately 3% resin in order to be as strongas 75 AFS with 1.5% resin.

Resin content is also related to sand density. Zircon, a silicate ofzirconium, ZrSiO is approximately 2 times as dense as silica sand.Therefore, only /2 as much resin is required by comparison to silicasand in order to obtain a resin coating of equivalent thickness on therefractory grains. For example, 150 AFS zircon is used in' accordancewith the present invention with 0.7l% resin; while with AFS silica sand,1.5% resin is required for similar strength properties.

In general, as the sand particle size decreases, e.g., as grain finenessincreases, mold permeability decreases. Also, excess resin content willfurther decrease mold permeability. Therefore, with very fine sand,e.g., AFS grain size, the resin content is kept as low as possible. Goodpermeability is essential for proper flow of molten metal into thinsections. Air or entrapped gas in the mold cavity reduces the effectivemetal head. Metal flow velocity in the mold is a function of the metalhead. Therefore, a reduction in metal head reduces the velocity of metalflow and, thus, for a given fluid life, the metal will not fiow as far.The result is improperly filled mold cavities. In other words, fluiditywould be decreased in a mold of low permeability compared with that in amold of high permeability.

In accordance with the present invention, the resincoated sand is blowninto the liner cavity with a pressure suflicient to completely fill theliner cavity, generally at pressures of approximately 50 to 80 p.s.i.Simultaneously with the blowing operation, the back-up member andpattern forming the shell cavity are vibrated to assist the resin-coatedsand to completely fill the cavity, especially for intricateconfigurations, and to aid in properly tamping the sand to effect auniformity in density of the shell. Vibration of the pattern and backupmember may be accomplished by mechanical vibration of the piston type orrotating ball type having frequencies within the range of 6,000 to 7,000cycles per minute when the sand-blowing pressure is within the range of50 to 80 p.s.i. Sonic and electronic vibrators of equivalent energiesmay also be used.

Applicants have discovered that the duration of the vibration cycle isimportant and must be shorter than the time necessary to cure thethermosetting resin coating the sand. Otherwise, rough surface on theshell will result. Generally, the vibration cycle begins with the startof the blow and is continued after the termination of the blow until theresin is only partially cured. This vibration cycle is longer than theblowing cycle but shorter than the curing time of the resin, preferablywithin the range of 1 to 30 seconds.

Prior to the blowing and vibration step, the patterns are preheated inthe range from about 350 to 550 F. Since phenolic resins will meltquickly prior to cure, excess heat is avoided. If the resin melts tooquickly, this minimizes the distance the sand can freely travel. Inorder to blow and fill complex thin liner cavities with the sand andresin mixture, it is often necessary to lower the preheat temperature to350 to 400 F. By so operating, baking times are approximately 1 minute.The longest baking time has been found to be about 2 minutes. At hightemperatures, baking times are so low at seconds.

Bake or cure time is dependent upon a particular phenolic resin. Thehigher melting point resins provide increased flow time for the moldmixture before melting and curing starts. However, the total curing timeis longer for the higher melting resins. Therefore, a compromise isusually, made between the melting point of the resin, and liner mixtureflow-ability in order to optimize the shortness of the cure cycle. Inaccordance with the present invention, it preferable to use resinshaving a melting point of about 220 F. When so operating, cure times ofto 30 seconds are obtainable, using preheat temperatures of about 400 to500 F.

Ceramic additives can be used to conduct heat away from and out of thecast molten metal at varying rates. Zircon sand is an example of a highconductivity ceramic that can be used. In an extension of this aspect ofthe invention, metal fibers can be used. In this extension, excessthermal conductivity is to be avoided because fluidity of the metal willbe unduly reduced. Therefore, a balance is desired between fluidity andhigh conductivity to give an increase solidification rate to give goodsurface finish and casting soundness.

However, the shell-lined molds of the present invention invention doprovide a fast cooling rate by conduction through the liner, byradiation because of the high permeability of the shells and byconduction through the backing members. This results in good castingsoundness with the casting having a fine grain size. Finer grain sizegenerally results in increased strength.

In accordance with the present invention, vacuum assist is employedduring metal pouring to remove air and gas from the mold cavity and thuseffectively increase metal head. Vacuum in the range of up to 21.75inches of water has been applied in accordance with the practice of thisinvention. Commonly a vacuum equivalent .4 to about 8 inches of water isemployed. This magnitude generally produces a significant increase influidity of the molten metal. Vacuum has been applied in two ways:

(1) Continuous aspiration.-An evacuated tank is attached directly to themold. In pouring, the mold is sealed and evacuated ahead of the metalstream.

(2) Vacuum is applied after start of metal pour. An evacuated tank isattached to the mold. When metal is poured, a switch is eithermechanically or electrically actuated by the metal stream, which opens avalve and applies vacuum to the mold.

Generally a static metal head of 4 inches is employed. Added to this isthe effective metal head from vacuum application. For example, if 2inches of water vacuum is used, this corresponds to 0.95 p.s.i. For ahigh temperature alloy with a density of 0.3 pound/inch 0.95 p.s.i. isequivalent to a metal head of 3.18 inches. Accordingly, the total headwould be 4+3.18=7.18. As mentioned above, vacuum on the order of 8inches of Water has been used. For the above example, this is equivalentto a metal head of 25.6 inches and a total head of 29.6 inches.

Metal pouring temperatures are generally the same as used for othertypes of molds. For example, nickel base alloys are usually pouredbetween 2850 and 3150 F. Ductile iron is poured from 2450 to 2650" F.

In accordance with the present invention and using standard C-lO silicasand AFS grain fineness number), surface roughness (smoothness) is inthe range of 70-400 microinches throughout the area cast against theshell-lined mold. Normal sand casting produces a surface roughness of upto 1,000 microinches.

An important advantage of the use of rigid molds is that dimensionalcontrol is greatly improved. Dimensional repeatability, by using thepresent invention, has been held within 0.002" on jet engine nozzlevanes.

A characteristic of resin-bonded shell molds is expansion on heating.This expansion has been measured to be up to 0.006/ inch at 450 F. Inaccordance with the present invention, a solid metal backing minimizesthis thermal expansion and contributes substantially to betterdimensional control.

The backing members also allow very close alignment between mold halvesand significantly increase dimensional control because of therepeatability of accurate V alignment.

By the present invention, rapid pouring is utilized in order to minimizetemperature loss. Pour times for small castings generally range from 0.5to 2 seconds.

It is to be understood that this invention is subject to logicalextension as will be evident to those skilled in the art. Also, theterminology employed is for the purpose of description and not oflimitation.

I now claim:

1. A method of making permanently backed, shelllined molds comprising;

preparing a charge of free-flowing, dry, homogenous,

shell-forming material comprising sand coated with a thermosettingresin,

positioning a pattern in close proximity to a backing member to from acavity therebetween,

heating the pattern and backing member to a temperature above themelting point of the thermosetting resin,

pressure injecting said charge into said cavity for a time sufiicient tofill the cavity,

vibrating said pattern and backing member at high frequency during saidinjecting step and continuing said vibration until such time that theresin ispartially cured,

and completing the cure of said resin after termination of saidvibration to form a shell linear against said backing member of uniformdensity, high surface smoothness and high porosity.

2. A method as claimed in claim 1 in which said charge consists ofessentially sand having a grain fineness numher in the range of 90 to180 AFS and a resin binder in the range of about 0.7 to 3 percent byweight.

3. A method as claimed in claim 1 wherein the sand has a grain finenessof 130 AFS and the resin content is less than 1.5 percent.

4. A method as claimed in claim 1 wherein the sand is Zircon having agrain fineness in the range of 105 to 150 AFS and the resin content ispresent in the range from 0.7 to 1 percent by weight.

5. A method as claimed in claim 1 in which the pattern is heated to atemperature Within the range of 350 to 550 F.

6. A method as claimed in claim 1 wherein the charge is injected intothe said cavity at a pressure in the range of 30 to 120 p.s.i.

7. A method as claimed in claim 1 in which the mold is vibrated for aperiod of from 1 to 30 seconds beginning with the sart of the injectingstep.

8. A method as claimed in claim 1 in which the thermosetting resin iscompletely cured by baking the shelllined member at a temperature of 350to 550 F. for a period of 15 to seconds.

References Cited UNITED STATES PATENTS I SPENCER OVERHOLSER, PrimaryExaminer.

EUGENE MAR, Assistant Examiner.

US. Cl. X.R.

